Document imaging systems (e.g., copiers and facsimiles) often represent documents as digital data which is used for reproduction or transmission of an original document. Comparing the intensity level of an element of the image (e.g., a pixel) to a threshold level yields binary data. If the intensity level exceeds the threshold, the element is represented by a binary value indicating white. If the intensity level does not exceed the threshold, the element is represented by a binary value indicating black. Problems occur when the document contains both text and images (e.g., photographs). Images typically contain a range of intensity values (i.e., gray scale) which cannot be preserved by the threshold process. Lines and line art can also present a problem for the threshold process. Lines can yield intensity levels which exceed the threshold in some image locations and do not exceed the threshold in other locations, depending on the width and orientation of the lines with respect to image elements. As a result, portions of the line in the processed image can be lost.
FIG. 1 illustrates the use of a conventional image processor for reproduction of documents. An original document 10 containing areas of text (i.e., characters) 12, images 14 and line features 16 is scanned by a document scanner 18. The scanner 18 generates a multi-level signal which is processed by an image processor 20. The image processor 20 processes the multi-level signal and provides a processed multi-level signal to a laser printer 22 or other output device which generates a reproduction 24 of the original document 10.
It is advantageous to process document areas containing characters and lines differently from document areas with embedded images. For example, character and line areas can be processed to enhance contrast and image areas can be processed to enhance image quality. FIG. 2 illustrates a document imaging system 30 described in Japanese Patent Publication No. 58-3374 utilizing two distinct image processing means. A multi-level signal representative of the document is received by a multi-level signal processing means 32 and a dither processing means 34. The output of the multi-level signal processing means 32 is provided to a separation means 36 and a selection means 38. The output of the dither processing means 34 is provided to the selection means 38. The separation means 36 autodiscriminates between portions of the multi-level signal corresponding to text and portions corresponding to image information based on characteristics of the output of the multi-level processing means 32. The separation means 36 generates a selection signal based on the autodiscrimination and provides it to the selection means 38. The selection means 38 generates an output signal by combining portions of the outputs from the processing means 32 and 34 according to the selection signal.
FIG. 3 shows another document imaging system 40 described in U.S. Pat. No. 4,996,603. A multi-level signal representative of the document is received by a character/photo separation circuit 42, a fixed slice processing circuit 44 and a halftone processing circuit 46. The outputs of the fixed slice processing circuit 44 and halftone processing circuit 46 are received by a selection circuit 48. The character/photo separation (autodiscrimination) circuit 42 generates a selection signal based on characteristics of the unprocessed multi-level signal. The selection circuit 48 generates an output signal by combining portions of the outputs from the fixed slice processing circuit 44 or the halftone processing circuit 46.
The prior art document imaging systems illustrated in FIGS. 2 and 3 have several disadvantages. The systems require circuitry to implement the separation and selection processes. The output signals from the processing means 32 and 34 or processing circuits 44 and 46 must be synchronized. Therefore, the memory used in the processing means 32 and 34 or processing circuits 44 and 46 must be matched to insure equal delay. Also, prior art document imaging systems are typically optimized for a particular orientation. As a result, text printed on a page in a vertical orientation might not be recognized as text and, therefore, the text might be processed as an image. The processed document would then contain inferior quality vertically oriented text. A preferred text orientation can also create other difficulties. For example, it can be more efficient (i.e., higher document throughput) to scan a standard document so that the scan line (e.g., CCD linear array line) is parallel to the longer document dimension, especially where the document processing rate is limited by the paper translation rate or scanner translation rate. The horizontal document text then appears vertically oriented to the scanner and results in inferior quality text.